Drying of distended fibrous products



lJuly 20, 19.43. o. J. KRUEGER ETAL 2,324,519

DRYING OF DISTENDED FIBROUS PRODUCTS 'Filed mig. s. 1940 Y 2sheets-sheet 1 July- 20, 1943. o. .1. KRUEGER ETAL f 2,324,519

DRYING OF DISTENDED FIBROUS ERODUCTS Filed Aug. s, 1940 V 2 sheets-sheetg Y l. v t Patentedlllly 2,0'

I DRYTNG oF nIs'rENDED mnous raonnc'rs Otto J. Krueger. Western Springs,andjohn Careslo, Chicago, lll., 'asllgnors to Masonite (llorporation, acorporation of Delaware Abpucauon august s, maserati No. 351,030

4 Claims.

'I'he invention relates to the drying of distended fibrous products, andrelates more particularly to the drying of such products made fromfibrous materials suchas kraft pulp, having for example foaming agentssuch as saponin mixed therewith and the mixture agitated with provisionfor air supply, sewing to produce 'a condition wherein relatively stablebubbles are incorporated with the fibers of the pulp in such manner asto produce distention. and maintain the ilbres in distended relation, inwhich relation' they are xed by drying.

The principal object of the invention is the provision of an improvementin the drying of distended fibrous products lby means of' heated air andproducts of combustion whereby an improved product can be made, andincreased speed of'production obtained. i

When relatively thick sheets, as for-example,

sheets one inch thick of distended fibrous material andfcontaining aconsiderable proportion of water are traveled along on a reticulatedsupport, asa Fourdrinier screen,.with excess or free water leaving thesheet from the bottom, the content of water retained in the sheet isleast at and near the top surface, and the water content increasesdownwardlyand is greatest at and near the bottom of the sheet.

which the lower' part of the sheetgets'from the carrying belt;blanketing of this upper partof vthe sheet by water vapors is less thanfor the Alower part of the sheet because there i-s less wav ter presentto be vaporized; and heat conducted lfrom theupper surface of the sheetdownwardly is decreased relative to the'corresponding eiect at and nearthe bottom of the sheet, because of v decreased heat conductivity duetoreducedway ter content.

` Consequently in the htfairdrying. of light weight sheets of distendedfibrous materials, it has been necessary to keep the temperaturesv nohigher than can be withstood by -the upper surface of the sheet without-charring or burning, i and this in turn has made it necessary to keepproduction speed correspondingly low or to make use oi very long andhighly-expensive ,drying 1 apparatus. These diiilculties are avoided bythe prior to its introduction into the drier.'

present invention thrqugh the addition'of water to the top surface andupper -part of the sheet By the addition of such water the temperaturesare no longer limited by the ready charring. of J the upper surface, asis the case without the add` 4ed water. A small proportion of ilreretarding material, such as boris acid for example, can ad- 7vantageously be dissolved in and supplied to the No great difficulty isencountered in drying the lower part of such a sheet by passingitthrough a hot air drier and subjecting it therein to temperaturessuiliciently high to secure rapid drying and good qualities in thesheet, without scorching or burning. The lower surface of the sheet isshielded to somev extent by the wire belt onwhich it is taken throughthe drier, and also by the roller supports for supporting the belt. Itis also more or less protected by water vapor 'formed from the water inthis part of the sheet. The' water in'this part of the sheet appearstogive good heat conductivity. so that-.the lheat is effectivelyconducted within` the interior of this part of the sheet, and the dryingextends'upward- Ily into the sheet from beneath.

If, however, thesheet is dried in a comparatively short drier, which canbe operated at high temperature, the y'saine speed of travel which givesgood results fat and near the bottom of the sheet because ofconsiderations as above'described will cause the upper surface of thesheet tobe charred or even burned, and at and about themedian zone ofthe sheet the drying'will'not be complete,

This is apparently due lto the relatively .low water content in theupper part of the sheet. The upper part of the sheet lacks the shieldingtop of the sheet with such added water, but good results can be obtainedwith or` without such dissolved soluble material. It might be supposedthat such addition of water or water containing boric acid orother fireretarding material would slow down the drying and increase the fuel costof drying, but'the reverse of this is really the case. In the drying ofthe sheet, water vapor is developed rapidly at the top surface of thesheet to which added water has been supplied, and this vapor appears tohave a protecting effect against ready chai-ring. The heat penetratesdownwardly` into the interior of the sheet or board, as is evi- `dencedby thorough drying throughout the sheet.

This effect appears to be produced by the greater heat'conductivity dueto the presence of the water added to :the sheet from above;- The fuelcost can be reduced, notwithstanding the fact that more water' has to bedried out. and this again appears to lbe'due to the added water by whichheat is conducted `i`nto the interior of the board from above.' suchheat conduction effect and the drying 'of the inner part of the sheetbeing the more efllcient 'since eil'ective heat conduction to theinterior of the sheettakes place not only from the bottom` ofthesheetupwardly. but alsoA downs. wardly fromthe top of thesheet.

It is-'thus an object of the invention to secure uniform drying of thesheets of distended fibrous material through the indirect, butnevertheless effective route by adding to the quantity of water whichhas to be dried out ofr the sheet.

The accompanying drawings forming a part y of the specification show anillustrative embodiment of apparatus for carrying out the invention. Insaid drawings,

Fig. 1 is a schematic side view of forming apparatus and also showing aportion of the' drying apparatus;

Fig. 2 is a side view on an enlarged scale of spraying apparatus;

Fig. 3 is a cross-section on line 3-3 of Fig. 2;

Fig. 4 is a side view of a part of a drier; and

Fig. 5 is a cross-sectional view on line 5-5 of Fig. 4.

Referring first to Fig. 1, reference character I0 designates a flow boxto which the mix, consisting preferably of fiber, foaming agent, sizingmaterials, water and air from which the sheet I is to be formed, issupplied through the pipe I2. This mix is kept in agitation by anagitator I4, located behind theintermediate dam I6 in the flow box, andafter passing over the dam I6 the mix is further agitated by the rollagitator 28. The mix from the ow box is delivered ,through the passageI8 on to the Fourdrinier Wire 22, the depth of the layer so deliveredbeing regulated by a slice plate 24, which can be adjusted for heightand for thickness of sheet, as by the regulating screws 26. The upperrun 21 of the Fourdrinier wire 22 is supported in the usual manner bymeans of the spaced rollers 28 shown only in part but providedthroughout the length of the Fourdrinier wire belt, and the sheet formedfrom the mix is designated by reference character 30. At the respectiveends of the forming apparatus the Fourdrinier wire 22 runs around abreast roll 32, and a couch roll 34. The return or under-run 38 of theFourdrinier wire 22 runs over rollers 38 and beneath other rollers as48, which may be made adjustable and used for tensioning the Fourdrinierwire as indicated by the arrows 48' placed near these rolls. Suctionboxes 4I, 4| may be provided, if desired, for compacting the extremelower portion of the sheet 38 somewhat in order to give increasedstrength to this part of the sheet. The roll 34 is preferablyperforated, and air under light pressure supplied at the interior'thereof to ensure that the sheet 38y is freed from the Fourdrinier wire22.

Transfer means are provided for delivering the formed sheet from theFourdrinier wire to the drier belt, aslit passes oi the Fourdrinier wirebelt and over the couch roll 34, and may consist, in the simplest form,of a short transfer belt 42, running over the rollers 44, 44, 44arranged in triangular relationV so as to'guide the belt 42 to performthe transfer function.

'I'he drying mechanism is' illustrated only in part. This dryingmechanism, which is of the hot air. type and does the drying bycirculating heated gases including products of combustion, is preferablycomposed of several sections, the number ofv sections and total lengthof the drier varying with the conditions for the particularinstallation, and in the showing herein made, the first and. secondsections only are illustrated. Such drier comprises essentiallyaf longdrying chamber or tunnel 50, and heating and air circulating chamberswhich are preferably arranged partially over and partially alongside thedrying tunnel 58. In the form shown, the first drying section, indicatedgenerally by the reference numeral 52, contains a blower fan 54, and anoutlet Inlet stack 60 supplies air for combustion and delivery thereofto the gas burners G2. A number of such burners are arranged across thewidth of the drier. In the form shown five of such burners are provided,the same being regulated as to supply of .combustible mixture of gas andair by the thermostatically operated controls 10 and the valves 12. Asindicated by the arrows on Fig. 4, the passage of circulating gases isin the first drier section 52 counter to the direction of travel of thesheet 30. The means provided for travelling sheet 38 through the tunneldrier preferably comprises the wire belt 14, which may advantageously beof the double spiral, or hinged spiral type well known for such use, thesame being carried at the entrance end of the drier around the tail roll18, and at the opposite ends of the drier carried by, a correspondinghead roll, not-show n. The belt 14 for carrying the sheet through thedrier is fairly. substantial and the spirals thereof made of fairlyheavy wire, and such belt aifordsquite a considerable shielding to theunder side of the sheet 38, as it passes through the drier,- thisshielding effect being increased by the relatively closely spacedrollers 88 on which the upper run of the drier belt 14 is supported asit passes through the drier. The return run 82 of the drier belt 14 Aissupported by therollers 84, which however do not need to be so closelyspaced. The succeeding sections of the tunnel hot air drier which ispreferably of n the single pass variety, are preferably arranged todirect the circulating gases so that the same pass through the tunnel 50in the same direction as the sheet 38. As already stated, but one suchfurther section is shown, and same is illustrated and heated by the gasburners are supplied to av conduit 90 which has branches 92 and 94 tothe opposite sides of the tunnel 58, and gases are directed into thetunnel 50 by means of nozzles 96 and 98 arranged to supply said gasesabove and below the sheet 38 respectively.

Except as they are combined with the provision made for spraying water,preferably water containing la percentage oi.' soluble re arrestingmaterial, upon the sheet before it is dried, the particular constructionand arrangement of the Fourdrinier apparatus and the drying apparatusconstitute no part of the presentinvention, and various other forms ofapparatus suited for these purposes can be utilized.

'I'he water delivery means are located over the sheet 38 on theFourdrinier wireI and are located at such a distance from the entranceto the drier as to give opportunity for the water to even out andpenetrate into the upper part of the sheet 38 before it reaches thedrier. The preferred relationship is substantially as shown in Fig. 1.

The provision which is made for delivering the water, preferablycontaining fire resistant materials in solution, on the sheet ,may be ofvarious kinds, but the preferred form shown inthe drawing consists of atransverse pipe I 80, which is of relatively large diameter, so that itscontents other than lcut oil, as for example 4cut off during the timecan be held ,atK substantially uniform pressure throughout its length,and this pipe III is provided with a series of spaced'spray nozzles ill,Ill. Thesenozzles il! are arranged to secure a uniform delivery of theiluid'upon thev sheet when the pipe |09 is located relatively close tothe sheet SII.` These spray nozzles can be directed laterally anddownwardly as indicatedat I, Fig. 1. A convenient mode of securinguniformdistribui tion of fluid from each of the nozzles III, consists inthe formation thereof as 'simple spray nozzles centrally bored, at i,and selection thereof to give substantially' identical nozzle openingsvadapted for uniform fluid-delivery. A convenlent mode of testing lthefluid delivery is by supplying fluid to the nozzles at uniform pressure,

which is substantially the .pressure used in oper-A l are relativelymuch more weak less effective in .such undried portions than in ation,and causing each nomic todischarge'into agraduate for a selected periodof time, and selecting from the supply of nonies those Vwhich deliversubstantially equal quantities of water insuch time.

While it is not essential in all cases to supply dissolved ilreresistant materalswitli the spray water, itis preferred to incorporate asmallpercentage of such materialwith the water' to be sprayed on thesheet. The preferred material y Whens the sheet is first formed on theIi'ourdriniery for this purpose is boric acid which is not decom tionconsist primarily in making it possible to secure desirably higherlspeeds of travel of the sheet being formedl and dried, and theproduction of a good qualityacceptable,"ilnished sheet prod-uct at suchincreased production speeds.

The advantages secured will be clearly under- The portions ofthematerial which have gone through the drier without being' and Vfragilethan the remainder'of the sheet, and cause a considerable reduction inthe strength and resistivityl of the whole sheet.` If, for example, suchpartly' undried sheets are stackedon one another until any considerableweight is imposed on the lower sheets in the stack, the-undried partswill colf lapse, which, of course, ruins the sheet for commercialpurposes. The sizing material used, and preferably consisting of waxorresin sizes. are

the parts of the sheet which have been thoroughly driedinthe drier. I

' Without commitment to particular theory, it appears that thebeneficial effects of the spraying of the water onLthe sheet prior todelivery thereof to the drier may be explained as follows:

wire, it consists of an evenly distributed mixture of water, fiber andfine air bubbles. `As the sheet proceeds along thev Fourdrinier wire aconsiderable proportion of the water drains from it, passing through thewire mesh to drain pans beneath. When no spray is used, upon enteringthe drier the amount of water remaining in the sheet is least at andnear .the upper surface. This 'upper part of the sheet having low watercontent, has poor heat conduction properties.-

Y As a result of this vuneven distribution of water y in the sheet. theupper portion oi'` thesheet containing least water is very difficult todry by reason of its low heat conductivity.-

When the sheet with the water so unevenly distributed, that is to say,with much less water stood from the following. comparison of operationwith and without the water lspray respectively:

Using the water spray, the apparatus is oper-4 ated under speeda'ndtemperature etc.'conditions adapted to produce a good, acceptable sheetwhich is substantiallly bone dry at the exit end of the drier. Thesheets of distended fibrous material are very light, weighing only a fewounces per square foot. Dried with the use of A thewater spray, thesheet is light-colored and free from char.

lf, then, for testing theA effect of nthe water spray, the operation iscontinued. with no change that the water spray is temporarily that asection of sheet I0, perhaps 25 Vfeet in length, has passed through thedrier, it will be found that when that part ofthe sheet to which addedwater was not supplied issues from the drier, instead of being lightcolored and free from charring on theupper surface as before, this partof the sheet will havea charred surface, and

`the interior of the sheet, instead of being subi stantially bone dry,will contain sufficient moisture so as to be perceptibly damp to thetouch. Furthermore, the undried part of the sheet so produced, even ifthe sheet is subsequently furl ther dried, is defective and unacceptablefor purat and near the top than at and near the lower surface, isexposed to the hot gases in the drier, the sheet will dry very rapidlyon the extreme upper surface, which then becomes even lessheatconductive. 'Ihis further retards the pas-4 sage of heat into thesheetV from above to evaporate the water within the sheet, and upon con-Y tinued exposure to the hot circulating gases of the drier, thetopsurface willV scorch and .char before the water is removed from thecenter of the sheet throughapplication of heat from above.

While this is going on near the upper surface of the sheet, asubstantially eillcient drying is being performed at andI near the lowersurface of the sheet, which has relatively highheat conductionproperties due to presence of water. with such emcient drying operationtaking place at the lower part of sheet, any' tendency to scorch in thislower part is reduced by the sheet being protected by the spirallinkbelt ofthe drier and bythe relatively closely spaced rollers overwhich the belt is passed. Another factor which tends both to emicientdrying and protection from charring at the bottom of the sheet 4appearsto consist inthe further protection afforded; by thepresence of watervapor in subi stantial quantities as the'drying proceeds.

The conditions just described are present and the effects just describedare obtained when the sheetis passed into the drier without auxiliaryspray treatment of its upper surface. .Quite dif ferent conditions andresults are obtainedv when the spray is used, andthe explanationthereof.

appears to be as follows: The spray` creates on the upper surface andlin the upper portion of the sheeta condition with respect to watercontent generally similar to that :lust described as existing normallyat or naar the lower surface poses for which good uniform sheets aredesired.

thoroughly dried of the sheet. or in other words,.a more evendistribution of water from bottom to top of the sheet. 'I'he water inthe upper portion of the sheet makes it more highly heat conductive. Abetter and more rapid water removal from the interior of the sheet takesplace from above. Also, water vapor formed over the sheet during thedrying operation appears to protect the upper surface against thescorching and charring that would take place without use of the spray.

The inclusion of various fire retardant chemicals of which boric acid isan example, serves to increase the protection aiorded against char. Thequantity of water to be sprayed on lthe i sheet and the amount of boricacid or equivalent tire resisting material to be included will varyconsiderably under varying conditions and with different thicknesses ofsheet. However, it may be stated, for. example, that when making a sheeteight feet wide and substantially one inch thick, and operating theapparatus at a speed of approximately eight feet per minute with atemperature of approximately '150 F., in the rst drying zone, andprogressively lower tempera-y tures in subsequent zones, excellenteffects can be obtained by delivery of water to the upper sur face oi'the sheet at the rate of approximately 1.2 gallons per minute, and adesirable propor tion of boric acid for use therewith is in theneighborhood of from .2% to .5% on the quantity of water sprayed, or anaverage of approximately .35%. It is a feature oi' great advantage thatthe use of the spray as has been described permits use of much higherdrying temperatures than without the spray, and makes it possible tooperate at increased speed, giving greatly increased production of goodquality and commercially acceptable product with use of drying apparatuswhich would otherwise have to be operated at lower drying temperaturesand much lower speed and output.

We claim:-

l. Process of treating relatively thick wet sheet material containingfibers held in distended relationship by relatively stable bubbles,which comprises delivering added water to the upper part of the sheet,and then drying the sheet by directing hot circulating gases toward thesheet surface to obtain uniformand increased speed in drying of thesheet whereby the vfibers are substantially fixed in their distendedrelationship and improved strength obtained with substantially nocharring.

`2. The process of treating relatively thick wet sheet materialcontaining iibers held in distended relationship by relatively stablebubbles, which comprises delivering added water to the upper part of thesheet substantially uniformly throughout its area, and then drying thesheet by directing hot circulating gases toward the upper and lowersurfaces of the sheet to obtain uniform and increased speed in drying ofthe sheet whereby the ilbers are substantially fixed in their distendedrelationship with substantially no charring.

3. The process of treating relatively thick wet

